Embodiments of the present invention relate to analog circuits, and more particularly, to analog amplifier circuits with multiple inputs.
Differential amplifiers are ubiquitous building blocks in many analog systems. Many applications require linear amplification of multiple differential input voltages by a single amplifier, where addition and subtraction are some of the general functions performed on the multiple input voltages. Such summing and differencing functions find applications in analog filters and bi-directional signaling receivers, to name just a couple.
A prior art differential amplifier circuit for two differential input voltages is shown in FIG. 1. Input ports 102A and 102B comprise a first differential input port pair for receiving a first differential input voltage, and input ports 104A and 104B comprise a second differential input port pair for receiving a second differential input voltage. A differential output voltage is provided by output ports 106A and 106B.
Transistors 102A and 102B are nMOSFETs (Metal Oxide Semiconductor Field Effect Transistor) comprising a first differential pair, and nMOSFETs 104A and 104B comprise a second differential pair. Current sink 108 sinks bias currents from nMOSFETs 102A and 102B, and current sink 110 sinks bias currents from nMOSFETs 104A and 104B. A load 112 is connected to the drains of nMOSFETs 102A and 104A, and load 114 is connected to the drains of nMOSFETs 102B and 104B. These loads may be active loads.
The circuit of FIG. 1 may easily be generalized to an arbitrary number of differential input port pairs, and may be used for performing summation or differencing. However, for demanding applications the circuit of FIG. 1 may suffer from limited input common mode voltage range, limited common mode rejection, and low gain.